A study of genetic mechanisms underlying the fat tail phenotype in sheep: methodological approaches and identified candidate genes (review)

Fat-tailed sheep breeds comprise 25 % of the global sheep population and are widely distributed in Africa, Asia (A. Davidson, 1999), the Middle East (S.P. Alves et al., 2013), as well as in Russia (I.M. Dunin et al., 2013). The fat-tailed sheep breeds were valued since their fat was an important ingredient of national cuisine in many ethnic groups (C. Perry, 1995; A. Hajihosseinlo et al., 2015). To-date the customers prefer lean food and cut down the fat intake, and therefore the benefits of large fat tails of sheep have reduced their importance for food production (A. Nejati-Javaremi et al., 2007; M. Moradi et al., 2012). The development of genomic editing technologies (N.A. Zinovieva et al., 2019) makes it relevant to search for genes that determine the “fat tail” phenotype for the subsequent knockout without side effects on other valuable traits of the fat-tailed sheep breeds. This review summarizes the results of studies on identification of candidate genes associated with fat tail trait. Various methods are used to identify candidate genes, including search for selective sweeps (signatures of selection) based on the calculation of differences in allele frequencies (Fst values) or haplotypes frequencies between populations (hapFLK method) (M.H. Moradi et al., 2012; M.I. Fariello et al., 2013; C.M. Rochus et al., 2018); genome-wide association studies (GWAS) that require an availability of a phenotypic variability base for the studied traits of economic importance (S.S. Xu et al., 2017); analysis of copy number variation (CNV) that can alter gene expression due to deletion or duplication of genes in the regions of variation (C. Zhu et al., 2016; Q. Ma et al., 2017; V. Bhanuprakash et al., 2018); study of gene expression using RNA-seq technology based on transcriptome analysis using new generation sequencing technology (NGS) (W.A. Hoeijmakers, 2013). Summarizing the research results, the most significant candidate genes associated with the fat deposition of the tail of sheep are BMP2 and VRTN (Z. Yuan et al., 2017; S. Mastrangelo et al., 2018; Z. Pan et al., 2019); PDGFD (C. Wei et al., 2015; S. Mastrangelo et al., 2018); genes of the Homeobox family (D. Kang et al., 2017; A.A. Yurchenko et al., 2019; A. Ahbara et al., 2019); SP9 (Z. Yuan et al., 2017; D. Kang et al., 2017); WDR92 and ETAA1 (Z. Yuan et al., 2017; L. Ma et al., 2018); CREB1 (S.S. Xu et al., 2017; L. Ma et al., 2018); FABP4 (M.R. Bakhtiarizadeh et al., 2013; B. Li et al., 2018); PPARA , RXRA , KLF11, ADD1, FASN, PPP1CA and PDGFA (C. Zhu et al., 2016; Q. Ma et al., 2017). To search for candidate genes involved in the formation of a fat tail phenotype in the Russian sheep breeds a QTL mapping resource sheep population was established by crossing the long-fat-tailed Karachaev and the short-thin-tailed Romanov breeds, to perform a genome-wide association study.